1. Field of the Invention
The present invention relates to an exhaust purification system for an internal combustion engine. More specifically, the present invention relates to an exhaust purification system for an internal combustion engine that switches at a predetermined condition between a lean operation mode that sets the air/fuel ratio of the air/fuel mixture sufficiently to the leans side from stoich and a stoich operation mode that sets the air/fuel ratio of the air/fuel mixture to stoich or the vicinity thereof.
2. Related Art
Exhaust purification systems for internal combustion engines purify HC (hydrocarbons), CO (carbon monoxide) and NOx (nitrogen oxides) contained in the exhaust gas of the engine. Exhaust purification systems that purify the above-mentioned three-way components in exhaust gas by employing the reactions on various types of catalysts provided in an exhaust channel have become mainstream. Among catalysts for purifying exhaust gas, various catalyst having different functions have been proposed, such as oxidation catalysts (DOC (Diesel Oxidation Catalyst)), three-way catalysts (TWC (Three-Way Catalyst)), NOx storage-reduction type catalysts (NSC (NOx Storage Catalyst)), and selective reduction catalysts (SCR catalyst (Selective Catalytic Reduction Catalyst)).
The oxidation catalyst has an oxidation function of purifying HC and CO by causing the oxidation reactions of HC and CO to advance under exhaust gas with the air/fuel ratio of the air/fuel mixture set to leaner than stoich containing an abundance of oxygen (exhaust gas with lean air/fuel ratio). In addition, this oxidation catalyst also has a three-way purification function whereby the oxidation reactions of HC and CO and the reduction reaction of NOx advance with high efficiency simultaneously under exhaust gas in which the air/fuel ratio of the air/fuel mixture is set to stoich (exhaust gas with stoich air/fuel ratio). The three-way catalyst corresponds to a catalyst produced by adding an oxygen storage material (OSC material) to the above-mentioned oxidation catalyst, and when compared with the above-mentioned oxidation catalyst, the three-way purification window, i.e. air/fuel ratio width exhibiting the three-way purification function, is wider. This effect arises due to the width of the air/fuel ratio fluctuation in the catalyst relative to the fluctuation in air/fuel ratio before the catalyst decreasing from the oxygen storage effect of the OSC material.
The selective reduction catalyst reduces NOx under the presence of a reducing agent supplied externally such as NH3 or HC, or a reducing agent present in the exhaust gas. The NOx storage-reduction type catalyst stores NOx in the exhaust gas under exhaust gas with a lean air/fuel ratio, and reduces by way of a reducing agent the NOx stored under the exhaust gas with an equivalence ratio of stoich or richer than stoich. Exhaust purification systems for engines based on lean combustion such as a lean-burn type gasoline engine or diesel engine often use a catalyst called a DeNOx catalyst such as this selective reduction catalyst and NOx storage-reduction type catalyst, by combining with the aforementioned such oxidation catalyst or three-way catalyst, in order to maintain the NOx purification function under exhaust gas with a lean air/fuel ratio.
Patent Document 1 proposes an exhaust purification system combining, among the aforementioned such catalysts, the NOx storage-reduction catalyst and the three-way catalyst. This exhaust purification system sets the air/fuel ratio of the air/fuel mixture to stoich, prior to the NOx storage-reduction type catalyst reaching activation, and purifies the three-way components in the exhaust gas by way of the three-way catalyst mainly. In addition, this exhaust purification system sets the air/fuel ratio of the air/fuel mixture to leaner than stoich after the NOx storage-reduction type catalyst has reached activation, and purifies HC and CO with the three-way catalyst, as well as purifying NOx by way of the NOx storage-reduction type catalyst.
Patent Document 2 proposes, with regard to a system equipped with a three-way catalyst and a NOx storage reduction catalyst provided downstream of this three-way catalyst, technology for setting the air/fuel ratio to stoich in order to purify exhaust by way of the three-way catalyst, prior to the NOx storage reduction catalyst activating.
[Patent Document 1] Japanese Unexamined Patent Application, Publication No. 2011-149360
[Patent Document 2] Japanese Unexamined Patent Application, Publication No. 2009-293585